1. Field of the Invention
The present invention relates to catalysts, methods of making catalysts, and processes for polymerizing polyolefins.
2. Description of the Related Art
Ziegler-Natta type polyolefin catalysts, their general methods of making, and subsequent use, are well known in the polymerization art. Typically, these systems include a Ziegler-Natta type polymerization catalyst component; a co-catalyst, usually an organoaluminum compound; and an electron donor compound. There are many issued patents relating to catalysts and catalyst systems designed primarily for the polymerization of propylene and ethylene that are known to those skilled in the art.
A Ziegler-Natta type polymerization catalyst is basically a complex derived from a halide of a transition metal, for example, titanium, chromium or vanadium, with a metal hydride and/or a metal alkyl that is typically an organoaluminum compound. The catalyst component is usually comprised of a titanium halide supported on a magnesium compound complexed with an alkylaluminum. While much is known about Ziegler-type catalysts, there is a constant search for improvements in their polymer yield, catalyst life, catalyst activity, powder size and shape (morphology), and in their ability to produce polyolefins having certain properties.
For example, it has been observed that some Ziegler-type catalysts can be quite unstable both prior to and during polymerization. This instability results in a breakdown of the catalyst and the formation of fine particles, and in turn, a formation of a large amount of polymer fines (particles less than 125μ). While these catalysts may originally posses a large particle size and a narrow particle size distribution, the catalyst can fragment upon mixing or upon polymerization, to provide a surprisingly large amount of polymer fine particles and a broad fluff particle size distribution curve. It has been observed that some catalysts gradually produced more fines upon standing. These observations suggested that a break down in the catalyst and the generation of fine catalyst particles could be occurring either prior to or during the polymerization process.